In the fabrication of microelectronic components, optical systems are used to project the image of a mask onto a substrate which is typically a resist covered semiconductor chip or a resist covered semiconductor chip packaging substrate. Also, these optical systems can be used to directly ablate the image of the mask onto a substrate.
The image field of an optical system is generally curved. The degree of curvature is determined by the Petzval sum. To increase the throughput through lithographic tools, it is desirable to be able to project a large object field so that fewer exposures are required to form patterns on a microelectronic substrate. If a large object field were to be used with commonly available high resolution optical systems, the projected image will be in good focus only over a limited image field. Since microelectronic substrates, such as semiconductor chips and semiconductor chip packaging substrates are planar with increased resolution or numerical aperture the correction of the Petzval sum becomes more and more difficult.
One solution to this problem is to use a curved substrate which has a curvature corresponding to the field curvature of the optical system. This is a not a realistic solution for fabrication of microelectronic components, since these components are planar. Alternatively, a mask having a curvature which is the negative of the curvature of the optical system can be used to result in an output image field which is substantially flat. This solution is also not realistic since the creation of a curved mask would be costly and require special tooling to fabricate.
It is an object of the present invention to provide an optical system where a substantially flat and substantially aberration free image field is projected onto a substantially flat planar surface.
In accordance with the present invention, an optical system having the desired reduction ratio 1X for projecting a reduced image of a mask is provided. For this optical system, preferably a 1X projection optical subsystem is provided which projects an image of the substantially flat mask, wherein the image has the opposite curvature of the reduction NX subsystem and thereby the image field of the reduction subsystem is substantially flat.
U.S. Pat. No. 4,812,028 to Matsumoto described a reflection type reduction projection optical system for optical lithography. For the Matsumoto system consisting of a plurality of monocentric reflecting and refracting surfaces and at least one aplanatic refracting surfaces. The Petzval sum of all the aplanatic refracting surfaces together and of all the remaining reflecting and refracting surfaces is corrected independently of each other. In contradistinction, the Petzval sum according to the present invention, is corrected by compensation of the field curvature of the input subsystem with that of the output subsystem resulting in a field compensated design. Also, the input and the output subsystems are essentially separately corrected for the odd aberrations, such as coma and distortion. But the even aberrations, such as spherical aberration, astigmatism and field curvature are substantially corrected by compensation between the subsystems.
In the Matsumoto system, all refracting and reflecting surfaces except the aplanatic surface operate in substantially the 1X monocentric mode which is not true in the system of the present invention. Both the systems of the present invention and Matsumoto's system have an intermediate image. The systems of the present invention present a curved intermediate image preferably formed by a 1X input subsystem to serve as a curved input to an output NX subsystem. This approach reduces the burden of field correction in the refractor by presenting to the refractor a compensatory curved field.
It is another object of the present invention to provide an optical system having a flat image field wherein the optical system has a 1X subsystem which provides a curved image which is the object of a second NX subsystem. The output of the combined system is a substantially aberration free and flat image field with very high resolution.
It is another object of the present invention to substantially correct for odd aberrations separately in each subsystem.
It is another object of the present invention to substantially correct for even aberrations by compensating the even aberrations of one subsystem with even aberrations in the other subsystem.
These and other objects, features and advantages of the present invention will be readily apparent to those of skill in the art from the following specification and the appended drawings.